Ultrafiltration is a useful way to concentrate and purify various liquid compositions. Among such compositions are silver halide photographic emulsions. Ultrafiltration is used to remove alkali metal nitrates and other impurities formed in precipitation of the silver halide.
Silver halide emulsions normally are prepared in a batch process by mixing a silver nitrate solution with an alkali metal halide solution in a gelatin medium. The composition is then washed to remove soluble salts. One type of ultrafiltration process suitable for use with the present method is described in Research Disclosures Vol. 102, October 1972, Item 10208 and Vol. 131, March 1975, Item 13122, which are incorporated herein by reference. The soluble impurities, e.g., alkali metal nitrates, permeate through the ultrafiltration membrane and the permeate is discarded.
One problem, however, is that the membrane becomes progressively fouled during use and the rate of permeation decreases. As a consequence, a batch of silver halide emulsion treated with a fresh membrane and one treated with a used membrane will be subjected to different processing conditions, including reactant concentrations, residence time in the reaction zone, fluid level in the reaction vessel, etc. Having been subjected to different processing conditions, the silver halide emulsions prepared in different batches will not have identical physical and photographic characteristics. It is possible to clean the membrane and thereby partially restore its original flux rate. However, some debris remains after each cleaning and the membrane progressively degrades with repeated use until it is too clogged to be of use.
As a result, successive batches of photographic emulsion will experience concentrates that vary widely over the course of one reaction to the next. The concentrate varies because the unwanted salts are removed at different rates because the semipermeable membrane becomes progressively clogged. So, salts are initially rapidly removed thereby initially quickly lowering the salt concentration, but over time, the rate of change of salt concentration is much lower. However, if a membrane is changed in the middle of a reaction, the rate of concentrate will increase again. Such changes make it difficult to time the reactions and the varying changes in concentrate result is inconsistent quality of photographic emulsions.